Example of Flatmap function.

FlatMap.kt

/**
 * You can edit, run, and share this code. 
 * play.kotlinlang.org 
 */

fun main() {
  val cities1 = ArrayList<City>()
  cities1.add(City("Teresina"))
  cities1.add(City("Campo Maior"))
  cities1.add(City("Altos"))
  cities1.add(City("José de Freitas"))
  
  val piaui = State("PI", cities1)
  
  val cities2 = ArrayList<City>()
  cities2.add(City("Timon"))
  cities2.add(City("Caxias"))
  cities2.add(City("São Luís"))
  cities2.add(City("Imperatriz"))
  val maranhao = State("MA", cities2)
  
  val listStates = ArrayList<State>()
  listStates.add(piaui)
  listStates.add(maranhao)
  
  val teste = listStates.flatMap {state-> state?.cities?.map {city-> "(${state.uf}) ${city.name}"}}
  println(teste)
}

data class City (var name: String)

data class State(var uf: String, var cities: List<City>)